In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
Photoconductive layers useful for xerographic purposes may be homogeneous layers of a single material such as vitreous selenium or may be composite layers containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated by U.S. Pat. No. 3,121,006 to Middleton and Reynolds which describes a number of binder layers containing finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. Such binder layers usefully contain particles of zinc oxide uniformly dispersed in a resin binder and is coated on a paper backing.
In the binder systems described in Middleton et al, the binder comprises a material which is incapable of transporting injected charge carriers generated by the photoconductor particles for any significant distance. As a result, such particles must be substantially in continuous particle-to-particle contact throughout the layer to permit sufficient charge dissipation for a stable cyclic operation. The uniform dispersion of photoconductor particles described in Middleton et al, therefore, requires a relatively high volume concentration of up to about 50 percent or more by volume of photoconductor in order to obtain sufficient photoconductor particle-to-particle contact for rapid discharge. It has been found, however, that high photoconductor loadings in the binder layers of the resin type also result in destruction of the physical continuity of the resin and significantly reduce the mechanical properties of the binder layer. Layers with high photoconductor loadings, therefore, are often characterized by a brittle binder layer. On the other hand, when the photoconductor concentration is reduced appreciably below about 50 percent by volume, the discharge rate is reduced, making high speed cyclic or repeated imaging difficult or impossible.
In a second Middleton et al patent another type of photoconductor is taught which utilizes a two phase photoconductive binder layer comprising photoconductive insulating particles dispersed in a homogeneous photoconductive insulating matrix. The photoconductor is in the form of a particulate photoconductive inorganic crystalline pigment broadly disclosed as being present in an amount from about 5 to 80 percent by weight. Photodischarge is believed caused by the combination of charge carriers generated in the photoconductive insulating matrix material and charge carriers injected from the photoconductive crystalline pigment into the photoconductive insulating matrix.
In U.S. Pat. No. 3,037,861 of Hoegl et al it is noted that polyvinyl carbazole exhibits some long-wave U.V. sensitivity such that its spectral sensitivity can be extended into the visible spectrum by the addition of dye sensitizers. Hoegl et al further indicates that additives such as zinc oxide or titanium dioxide may be used in conjunction with polyvinyl carbazole as a photoconductor.
Furthermore, certain specialized layered structures have been proposed for reflex imaging. For example, U.S. Pat. No. 3,165,405 to Hoesterey utilizes a two-layered zinc oxide binder structure for reflex imaging. In Hoesterey two separate continuous photoconductive layers having different spectral sensitivities are utilized to carry out a reflex imaging sequence. The Hoesterey device utilizes the properties of multiple photoconductive layers in order to obtain the combined advantages of the separate photoresponse of the respective photoconductive layers.
Although the above patents rely upon distinct mechanisms of discharge throughout the photoconductive layer, they generally suffer from common deficiencies in that the photoconductive surface during operation is exposed to the surrounding environment, and particularly in the case of cycling xerography, susceptible to abrasion, chemical attack, heat, and multiple exposures to light during cycling. These effects are characterized by a gradual deterioration in the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, localized areas of persistent conductivity which fail to retain an electrostatic charge, and high dark discharge.
In addition to the problems noted above, these photoconductive layers require that the photoconductor comprise either a hundred percent of the layer, as in the case of the vitreous selenium layer, or that they preferably contain a high proportion of photoconductive material in the binder configuration. The requirements of a photoconductive layer containing all or a major proportion of a photoconductive material further restricts the physical characteristics of the final plate, drum or belt in that the physical characteristics such as flexibility and adhesion of the photoconductor to a supporting substrate are primarily dictated by the physical properties of the photoconductor, and not by the resin or matrix material which is preferably present in a minor amount.
Another form of composite photosensitive layer utilizes a layer of photoconductive material covered with a relatively thick plastic layer and coated on a supporting substrate.
U.S. Pat. No. 3,041,166 of Bardeen describes such a configuration in which a transparent plastic material overlays a layer of vitreous selenium contained on a supporting substrate. The plastic material, as described, has a long range for charge carriers of the desired polarity. In operation, the free surface of the transparent plastic is electrostatically charged to a given polarity. The device is then exposed to activating radiation which generates a hole-electron pair in the photoconductive layer. The electron moves through the plastic layer and neutralizes a positive charge on the free surface of the plastic layer thereby creating an electrostatic image.
French Pat. No. 1,577,855 to Herrick et al also describes a special purpose composite photosensitive device adapted for reflex exposure by polarized light. One embodiment employs a layer of dichroic organic photoconductive particles arrayed in oriented fashion on a supporting substrate with a layer of polyvinyl carbazole formed over the oriented layer of dichroic material. When charged and exposed to light polarized perpendicularly to the orientation of the dichroic layer, the oriented dichroic layer and polyvinyl carbazole layer are both substantially transparent to the initial exposure light. When the polarized light hits the white background of the document being copied, the light is depolarized, reflected back through the device and absorbed by the dichroic photoconductive material. In another embodiment, the dichroic photoconductor is dispersed in oriented fashion throughout the layer of polyvinyl carbazole.
While the Bardeen and Herrick concepts are very advantageous, the fact remains that the inherent brittleness of the most efficient inorganic photoconductors continues to limit effective usage where high speed belt-type photoreceptors are needed.